Low pressure chemical vapor deposition (LPCVD) processes for the deposition of silicon nitride layers are well known. However, such films have been processed only in batch-type processing chambers that can process up to about 100 substrates (silicon wafers) per batch at fairly low pressures of about 300 millitorr. The deposition rate is quite low, e.g., about 30-40 angstroms per minute, but economies of scale are achieved by processing a plurality of substrates at one time.
However, as semiconductor substrates have become larger, up to 6-8 inches in diameter, and the number of devices formed in a substrate has increased, substrate processing is increasingly being performed in single substrate chambers. Single substrate processing can be performed in much smaller processing chambers, and the processes can be better controlled. Further, vacuum processing systems have been developed to carry out more than one processing step on a single substrate without removing the substrate from a vacuum environment. The use of such systems results in a reduced number of particulates that contaminate the surface of substrates during processing, thereby improving device yield. Such vacuum systems include a central robotic transfer chamber connected to various processing chambers, such as the Applied Materials 5000 series processing system described in U.S. Pat. No. 4,951,601 to Maydan et al.
A LPCVD chamber useful herein will be described with reference to FIG. 1. A single substrate reactor 31 has a top wall 32, side walls 33 and a bottom wall 34 that define the reactor 31 into which a single substrate 35, such as a silicon wafer, can be loaded. The substrate 35 is mounted on a pedestal or susceptor 36 that is rotated by a motor 37 to provide a time averaged environment for the substrate 35 that is cylindrically symmetric. A preheat ring 40 is supported in the chamber 30 and surrounds the wafer 35. The wafer 35 and the preheat ring 40 are heated by light from a plurality of high intensity lamps 38 and 39 mounted outside of the reactor 31. The top wall 32 and the bottom wall 34 of the chamber 30 are substantially transparent to light to enable the light from the external lamps 38 and 39 to enter the reactor 31 and heat the susceptor 36, the substrate 35 and the preheat ring 40. Quartz is a useful material for the top wall 32 and the bottom wall 34 because it is transparent to light of visible and IR frequencies; it is a relatively high strength material that can support a large pressure difference across these walls; and because it has a low rate of outgassing.
During deposition, the reactant gas stream flows from a gas input port 310, across the preheat ring 40 where the gases are heated, across the surface of the substrate 35 in the direction of the arrows 41 to deposit the desired films thereon, and into an exhaust port 311. The gas input port 310 is connected to a gas manifold (not shown) that provides one or a mixture of gases to enter the reactor 31 via a plurality of pipes into this port. The locations of the input ends of these pipes, the gas concentrations and/or flow rates through each of these pipes are selected to produce reactant gas flows and concentration profiles that optimize processing uniformity. Although the rotation of the substrate and thermal gradients caused by the heat from the lamps 38 and 39 can significantly affect the flow profile of the gases in the reactor 31, the dominant shape of the flow profile is a laminar flow from the gas input port 310 and across the preheat ring 40 and the substrate 35 to the exhaust port 311.
It would be desirable to be able to deposit uniform, thin films of silicon nitride on semiconductor substrates in a single substrate processing chamber at a practicable deposition rate.